CO_2 Methanation over Ru/TiO_2 Catalysts under UV Irradiation and Heating

CO2 hydrogenation was carried out over Ru/TiO2 catalysts prepared by wet impregnation method. It was found that CO2 methanation(CO2 + 4H2 → CH4 + 2H2O) could be catalyzed by Ru/TiO2 both under UV irradiation and heating. Results showed that CH4 was the only C-contained product under UV irradiation in temperature range from 150–350℃, while both CH4 and CO(minor product) were formed at temperature higher than 400℃ under heating conditions. This indicated that reaction temperature affected obviously on selectivities of products. For all the catalysts with different Ru loadings, CO2 conversion increased firstly and then decreased from 150℃ to 550℃, and corresponding maximum value of CO2 conversion of 77.58% was achieved at 350℃ over 1.5wt% Ru/TiO2 catalysts. Further increasing of reaction temperature over 400℃ led to the increase of CO selectivity. Based on reaction data, XRD, XPS and N2 adsorption-desorption characterization results, we found that CO2 hydrogenation occurred in different ways under UV irradiation and heating conditions(150–550℃). Under UV irradiation, excited electrons were firstly trapped by Ru site and then these electrons reduced CO2 species, which was adsorbed on Ru surface to form CH4 via Ru C intermediate. However, under heating condition(150–550℃), H2 was first decomposed into H atoms over Ru site, then the adsorbed CO2 species on Ru site was reduced by H atoms to form Ru C intermediate. Finally, Ru C reacted with H atoms to produce CH4. Although CO2 hydrogenation was taken place via same intermediate under UV irradiation and heating conditions, the Ru C intermediate was formed in different ways, i.e., the activation of CO2 was induced in the different routes, therefore the different selectivities of products were obtained under UV irradiation and heating at temperature higher than 400℃ consequently.